Roger Federer Beats Gael Monfils in U.S. Open Quarterfinal

Five-Time Champion Saves Two Match Points to Book Place in Semifinals

NEW YORK—Fans at Arthur Ashe Stadium had seen this before: the errors, the wild forehands, backhand shanks and silly drop shots. What was Roger Federer doing?

After a fantastic summer—final at Wimbledon, final in Toronto, victory in Cincinnati—the 33-year-old was fumbling his way through a U.S. Open quarterfinal against Gael Monfils. The sentimental favorite, who used to own this tournament but hasn't won the title since 2008,...